1. Field of the Invention
This invention pertains to certain means of counteracting the negative effects of mechanical resonances on the operation of stepper motors. More particularly, it concerns the reduction of the effects on stepper motors of resonant wave(s) in a machine or device to which a stepper motor is attached. As used hereafter, the term "stepper motor" refers primarily to the "hybrid" type stepper motor; however, the same principles would also apply to the permanent magnet, variable reluctance, canstack, axial airgap and other types of stepper motor designs where the stator is mechanically affixed to the outer motor housing.
Such waves are the natural result of mechanical resonance(s) in any particular machine or device. These mechanical waves of energy, when coupled to a stepper motor's rotor assembly (through the motor output shaft) and/or stator assembly (through the motor's case) can substantially increase the amount of energy required for a stepper motor to continue its rotational movement from one magnetic pole to the next. The result can range from an increase in the electrical energy required by the stepper motor for the same amount of output torque (at a given speed), to the complete stalling of the stepper motor.
The present invention consists of an apparatus and method for counteracting the negative effects on the operation of a stepper motor of mechanical resonances in a particular machine or device. The primary results of the present invention is a substantial increase in a stepper motor's speed range and a significantly improved torque curve.
2. Description of the Background Art
It is well known in the field of stepper motors that each particular stepper motor will have one or more resonant points over its frequency range of operation. The size, mechanical construction, air gap and many other physical characteristics of a stepper motor vary over a wide range, thereby resulting in a divergence of inherent stepper motor resonances. There are also mechanical resonances in any machine or device to which a stepper motor may be attached.
When a stepper motor is attached to any machine or device, the stepper motor, along with such machine or device, become one system that will have its own unique mechanical resonances. These "system" resonances can cause problems with the operation of a stepper motor, including increased demand at certain operating frequencies for electrical energy (for the same torque output), to the complete stalling of the stepper motor.
To date, there have been very limited and rather unsuccessful efforts to counteract the negative effects of such stepper motor/machine system resonances. Rather, it is the custom in the industry to operate a particular stepper motor within a frequency range in which it is not seriously affected by any system resonance.
Another common industry approach to the problem of system resonances has been to use an isolation mounting technique similar to that used in the mounting of a variety of mechanical devices, including internal combustion engines, transmissions, and other types of electrical motors. This approach involves the partial separation of the stepper motor from the machine or device by means of a flexible standoff, usually made from rubber (or similar flexible material), that lowers the total energy of the mechanical waves moving towards or from the stepper motor in the direction parallel to the stepper motor's rotor assembly. Such isolation mounting devices operate within a limited frequency range (depending upon the type and shape of the isolating material), and are directed at reducing the amount of transmitted and reflected energy that is moving in a linear direction parallel with respect to the stepper motor's rotor; such types of isolation mounts do not significantly affect the amount of mechanical wave energy that is transmitted to or from the stepper motor in a direction that is perpendicular to the stepper motor's rotor assembly (i.e., mechanical waves moving energy clockwise or counterclockwise with respect to the motion of the rotating rotor assembly of the stepper motor).
Another common industry approach is the use of viscous dampers placed on the output shaft of the stepper motor. These are designed to act in a manner similar to a flywheel by temporarily smoothing out short term various in torque. However, such devices, like isolation mounts, do not directly deal with the problem of the mechanical waves interfering with the relationship of the stator assembly and the rotor assembly inside the stepper motor. They also have the disadvantage of adding drag to the output shaft at all speeds, thereby wasting energy. Lastly, they do not significantly improve dynamic speed range, but in fact, may decrease maximum speed capability.